It used to be that the nation’s most populous state found itself on the sidelines when Republicans and Democrats squared off in congressional elections. California’s carefully engineered districts were so safe for incumbents that only one of the 53 seats changed party hands during the last decade.

Then last year, a voter-authorized citizens commission took over the job of drawing district maps and shook up the political landscape. Now, the two parties and their allies are fighting it out in at least 10 newly competitive seats around the state.

Democrats need to pick up 25 seats around the country to wrest control of the House from Republicans — an achievement that most analysts say is unlikely. But vivid blue California offers Democrats some of their best prospects, and they think they have a good chance of defeating four GOP incumbents.